1. Field of the Invention
The present invention relates generally to control circuits and, more particularly, to a circuit for detecting the discharge state, or voltage level, of a battery, so that a lamp can be appropriately turned ON or OFF.
2. Description of the Background
FIG. 1 shows a circuit useful in energizing a metal halide lamp such as used in a camera. In FIG. 1, a DC voltage Vcc is supplied across voltage source terminals 6 and 6' and such DC voltage source might comprise a secondary battery of the camera and consist of a nickel-cadmium battery or the like. The DC voltage Vcc supplied by the secondary battery is consumed by the load, so that the fluctuation in voltage at terminal 6 is relatively large.
The voltage across terminals 6 and 6' is connected through a power switch 21 to a fixed contact 22a of a switch 22 and to a closed or "make" contact 27a of a relay 27. A movable contact 22b of switch 22 is electrically connected to a movable contact 27b of relay 27, to one end of a relay drive coil 24, to a cathode of a reverse current blocking diode 23, and to an input terminal 25a of an integrated circuit (IC) that operates as a voltage detecting circuit 25. Voltage detecting circuit 25 supplies a high-level signal at an output terminal 25b thereof when the DC voltage applied to input terminal 25a is higher than a predetermined voltage level, and supplies a low level signal at output terminal 25b when the DC voltage Vcc is lower than the predetermined voltage level.
Output terminal 25b of voltage detecting circuit 25 is connected to the base of a switching transistor 26, whose collector is connected to an anode of diode 23 and to the other end of relay drive coil 24. The emitter of transistor 26 is connected to ground, and the movable contact 27b of relay 27 is connected to one side of a metal halide lamp 19 that comprises the load in the circuit. The other side of metal halide lamp 19 is connected to ground. The other contact 27c of relay 27 is the open or "break" contact.
In the operation of the above-described control circuit, if the metal halide lamp 19 is to be turned ON the power switch 21 is closed and the switch 22 is manually depressed to the closed state. Switch 22 is ON or closed only so long as it remains manually depressed. At that time, the magnitude of the voltage Vcc supplied from the DC voltage source to input terminal 25a of voltage detecting circuit 25 is detected thereby. If the detected voltage falls within a predetermined voltage range, voltage detecting circuit 25 generates a high-level voltage at output terminal 25b, thereby biasing the base of switching transistor 26 so that switching transistor 26 is turned ON. Thus, relay drive coil 24 is excited by voltage Vcc and movable contact 27b resting on break contact 27c is moved into contact with make contact 27a, whereby the voltage Vcc from voltage source terminal 6 is supplied to metal halide lamp 19. Therefore, metal halide lamp 19 is turned ON, and a current flows through relay drive coil 24 causing relay 27 to keep itself pulled in.
Under this condition, if the voltage Vcc from the DC voltage source decreases to the predetermined, minimum acceptable value, a low-level voltage will be developed at output terminal 25b of voltage detecting circuit 25, whereby switching transistor 26 is turned OFF and the current flowing through the relay drive coil 24 is interrupted. Thus, movable contact 27b of relay 27 will return to break contact 27c and relay 27 will be turned OFF. If relay 27 is released from the pulled in state, the power to lamp 19 is interrupted, which is the same as power switch 21 being turned OFF.
According to the above-described arrangement, metal halide lamp 19 must be turned ON and OFF by use of manual switch 22, and voltage detecting circuit 25 for detecting whether the voltage of the DC voltage source has fallen below a predetermined value must be formed of an integrated circuit, which results in an expensive control circuit. Furthermore, a relay must be controlled, which results in a control circuit that consumes more power than is desirable.